... You can sell your phone to another person. They can go to AT&T and get a SIM for the iPhone...then go home and activate it with their account. Very similar to when you get back a refurbished phone or a replacement from an Apple Store (just a bit reversed)...

... You could sell your iPhone to me and before handing it over, the iPhone includes an option to erase all data and settings and you would remove the SIM card and transfer it to your new iPhone.

I would visit a local AT&T Store and request a new SIM card for the used iPhone I purchased which would allow me to activate this iPhone with my existing AT&T number or if I wasn't an existing AT&T subscriber I would need to become one via the iTunes activation process with AT&T...

So if you're an AT&T subscriber who's interested in the iPhone, you might be able to get a used 1.0 phone fairly inexpensively when the 2.0 phone comes out (perhaps in ?).